1. Field of the Invention
Embodiments of the invention are directed to pulsed-neutron density logging tools. More particularly, embodiments of the invention are directed to pulsed-neutron density logging tools that compensate for neutron transport effects.
2. Background of the Invention
Pulsed-neutron logging tools may be used in cased boreholes, and in some situations pulsed-neutron logging tools may be operated within production tubing. Pulsed-neutron logging tools operate on the principle of releasing high energy neutrons, on the order of 14 Mega electron Volts (MeV) into the formation. The high energy neutrons inelastically collide with other particles and thereby create gamma rays (known as inelastic gamma rays). Some of the inelastic gamma rays created by the collisions make their way back to, and are detected by, gamma ray detectors on the logging tool. The ratio of received gamma rays between a detector close to the pulsed-neutron source (the near detector) and a detector at some distance from the pulsed-neutron source (the far detector) may be indicative of the bulk density of the formation surrounding the borehole.
As neutrons lose energy through inelastic collisions (and the creation of inelastic gamma rays), they eventually reach an energy of thermal equilibrium, approximately 0.025 electron Volts (eV). When the neutrons approach this thermal equilibrium energy, they may be captured by surrounding atoms, and in the capture process a gamma ray may be produced (known as a thermal capture gamma ray). Thermal capture gamma rays too may propagate to the detectors on the tool.
When using a pulsed-neutron logging tool as a bulk density measurement device, inelastic gamma rays carry most of the information as to the formation bulk density. Thus, to determine a formation bulk density using a pulsed-neutron tool, it may be desirable to remove from the total received gamma rays the thermal capture gamma rays to be left only with inelastic gamma rays.
However, even after removing the thermal capture gamma rays from the total received gamma rays, bulk density measurements made with a pulsed-neutron logging tool may not closely match actual bulk density.